*::after, *::before {
  box-sizing: border-box;
}

*{
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  --bg: #222
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  background: var(--bg);
  gap: 20px;
}

.btn {
  position: relative;
  padding: 12px 40px;
  font-size: 1.05em;
  border-radius: 40px;
  background: #2f2f2f;
  color: #bbb;
  letter-spacing: .15em;
  text-decoration: none;
  text-transform: uppercase;
  user-select: none;
  transition: .5s;
  overflow: hidden;
}

.btn:hover {
  color: #fff;
}

.btn span {
  position: absolute;
  border: 2px solid #FFF;
  transform: translate(-50%, -50%);
  pointer-events: none;
  border-radius: 50%;
  animation: animate 2s linear infinite;
  filter: brightness(2);
}

@keyframes animate {
  0% {
    width: 0;
    height: 0;
  }
  100% {
    width: 350px;
    height: 350px;
  }
}